It Spreads Silently, Kills Slowly, and Just Arrived on the West Coast

A parasitic tapeworm capable of causing a fatal, cancer-like disease has been detected in wild animals along the West Coast for the first time, according to a new University of Washington study. Researchers found the parasite in 37 out of 100 coyotes tested in the Puget Sound region — a detection rate that surprised the scientists who conducted the study and prompted calls for expanded surveillance across the region.

The parasite is called Echinococcus multilocularis, commonly known as the fox tapeworm. When it infects a human or domestic animal, it can produce slow-growing cysts in the liver that behave like a metastatic tumor, spreading to surrounding tissue and, in some cases, to other organs. Without treatment, infection is often fatal. The World Health Organization lists the disease it causes — alveolar echinococcosis — among the most dangerous of the world's neglected tropical diseases, and considers it the third most important food-borne illness globally.

What the Study Found

Researchers from the University of Washington collected samples from 100 coyotes in the Puget Sound area and found 37 carrying the tapeworm — the first confirmed detection of E. multilocularis in a wild host anywhere on the contiguous West Coast. Infected animals were found across a wide geographic range within the Puget Sound region.

The findings were published in March 2026 in the journal PLOS Neglected Tropical Diseases.

"This parasite is concerning because it has been spreading across North America," said lead author Yasmine Hentati, who completed her doctorate in environmental and forest science at UW. "There have been numerous cases of dogs getting sick, and a handful of people have also picked up the tapeworm. The fact that we found it here in one-third of our coyotes was surprising, because it wasn't found anywhere in the Pacific Northwest until earlier this year."

How the Parasite Moves Through the Food Chain

E. multilocularis relies on a multi-host life cycle. Coyotes, foxes, and other canids serve as the primary hosts, carrying adult tapeworms in their intestines — sometimes thousands of them — without becoming ill. The worms release eggs that pass into the environment through feces, where they can survive in soil and on vegetation.

Rodents are the parasite's critical intermediate hosts. When small mammals consume food contaminated with coyote feces, the tapeworm eggs migrate to their livers and develop into cysts, eventually killing the rodents. Coyotes become reinfected when they eat those animals, completing the cycle.

Humans and domestic dogs are what researchers call accidental hosts. Infection occurs when someone ingests tapeworm eggs — typically through food or hands that have come into contact with contaminated feces from an infected dog or coyote. Once established, the disease progresses silently. Symptoms of alveolar echinococcosis may not appear for five to 15 years after initial exposure, which makes early detection difficult and often delays diagnosis until the disease has advanced.

The Centers for Disease Control and Prevention lists echinococcosis as a reportable disease and recommends that travelers and people who work with canids or wildlife in affected regions be aware of the risk.

A More Infectious Strain From Europe

The version of E. multilocularis now circulating in the Pacific Northwest is not the same strain that historically appeared in remote parts of Alaska. Genetic analysis shows that the earlier Alaska cases involved a tundra variant, while the current spread across North America — and the coyotes in the Washington study — carries a European-origin strain considered more infectious than its predecessor.

Scientists are not certain how this strain became established in North America. One hypothesis is that imported dogs were not required to undergo deworming before entering the United States or Canada. Another theory suggests the parasite may have arrived in red foxes brought to North America for hunting purposes more than a century ago.

What the Risk Is and What Dog Owners Should Know

The study authors are careful to note that a 37 percent detection rate in coyotes does not translate to significant human risk. Coyotes become infected by regularly eating the raw livers of rodents — a behavior that healthy domestic dogs typically do not share.

The University of Washington's announcement of the research emphasized that seven canine cases have been recorded across Washington, Oregon, and Idaho since 2023, with five in Washington state alone. No human cases have been reported on the West Coast. The risk to the general public is considered low.

However, dog owners in the Pacific Northwest — particularly those with dogs that hunt small mammals or scavenge carcasses — should be aware of the finding and discuss deworming options and parasite testing with their veterinarians. Co-author Guilherme Verocai, , an associate professor and director of the Parasitology Diagnostic Laboratory at Texas A&M's College of Veterinary Medicine and Biomedical Sciences, recommends routine veterinary parasite screening and preventative deworming as simple protective measures.

A dangerous tapeworm has arrived on the West Coast and awareness is the first line of defense. Share this story with dog owners and outdoor enthusiasts in your area, and keep your forecast in the now with Weather Forecast Now.